ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-13-4681-2013Heterogeneous ice nucleation on phase-separated organic-sulfate particles: effect of liquid vs. glassy coatingsSchillG. P.12TolbertM. A.121Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA2Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA0605201313946814695This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/4681/2013/acp-13-4681-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/4681/2013/acp-13-4681-2013.pdf

Atmospheric ice nucleation on aerosol particles relevant to cirrus clouds
remains one of the least understood processes in the atmosphere. Upper
tropospheric aerosols as well as sub-visible cirrus residues are known to be
enhanced in both sulfates and organics. The hygroscopic phase transitions of
organic-sulfate particles can have an impact on both the cirrus cloud
formation mechanism and resulting cloud microphysical properties. In
addition to deliquescence and efflorescence, organic-sulfate particles are
known to undergo another phase transition known as liquid–liquid phase
separation. The ice nucleation properties of particles that have undergone
liquid–liquid phase separation are unknown.
<br></br>
Here, Raman microscopy coupled with an environmental cell was used to study
the low temperature deliquescence, efflorescence, and liquid–liquid phase
separation behavior of 2 : 1 mixtures of organic polyols (1,2,6-hexanetriol
and 1 : 1 1,2,6-hexanetriol + 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol)
and ammonium sulfate from 240–265 K. Further, the ice nucleation efficiency
of these organic-sulfate systems after liquid–liquid phase separation and
efflorescence was investigated from 210–235 K. Raman mapping and
volume-geometry analysis indicate that these particles contain solid
ammonium sulfate cores fully engulfed in organic shells. For the ice
nucleation experiments, we find that if the organic coatings are liquid,
water vapor diffuses through the shell and ice nucleates on the ammonium
sulfate core. In this case, the coatings minimally affect the ice nucleation
efficiency of ammonium sulfate. In contrast, if the coatings become
semi-solid or glassy, ice instead nucleates on the organic shell. Consistent
with recent findings that glasses can be efficient ice nuclei, the
phase-separated particles are nearly as efficient at ice nucleation as pure
crystalline ammonium sulfate.